1. Field of the Invention
This invention relates, in general, to a denuder assembly for the collection and removal of soluble atmospheric gases and more particularly to a denuder assembly utilizing a wetted membrane and methods for its use.
2. Description of Related Art
Diffusion denuder techniques have largely replaced filter-based methods for the collection of atmospheric trace gases. The selective removal of gases from gas-particle mixtures by a denuder exploits the large difference in diffusion coefficients between gases and particles. When an air sample is drawn through a denuder under laminar flow condition, gas molecules of the air sample diffuse to the walls of the denuder and are captured, while very small particles diffuse too slowly for them to reach the wall. Although a variety of coated denuders have been used for the collection and removal of atmospheric gases, known processes utilizing such coated denuders have several disadvantages as such known processes are laborious, time-consuming, and contamination-prone as the processes involve coating, drying, and washing denuder tubes.
In contrast, wet denuders depend on direct gas-liquid contact. In known wet denuders, a sample gas flow is typically surrounded by a flowing collector liquid. The wettability of a denuder surface of a wet denuder is a critical factor in maintaining a thin liquid film exposed to the sampled air, which typically flows at a much greater rate and generally countercurrent to the liquid flow. Such continuously wetted denuders have been reported in the last 15 years. For example, Keuken et al. reported an early wetted denuder having an annular design and which rotates around its axis, operating in a horizontal configuration, in order to keep the annulus uniformly wet. See Menno P. Keuken et al, Simultaneous Sampling Of NH3, HNO3, HCl, SO2 And H2O2 In Ambient Air By A Wet Annular Denuder System, Atmospheric Environment, vol. 22, no. 11, pp. 2541-2548 (1988). With such an annular denuder, collection efficiency for many water-soluble gases is near-quantitative at sampling rates up to 32 L/min while the denuder shows <1% particle loss (for the particle size tested).
Other known wetted denuder systems operate vertically to avoid particle loss within in the denuder due to gravitational deposition. For example, Dasgupta et al. have described parallel plate wet denuders containing silica-coated wetted areas and a high flow (e.g., 50 L/min), and compact, multiple-parallel-plate denuder bearing eleven wetted polyester screens. See, e.g., Purnendu K. Dasgupta et al., A Multiple Parallel Plate Wetted Screen Diffusion Denuder For High-Flow Air Sampling Applications, Analytical Chemistry, vol. 69, no. 24, pp. 5018-5023 (1997). Jaeschke et al. have reported a tubular glass wet denuder in which the wetted denuder surface includes a nylon stocking material as the wall lining. See W. Jaeschke et al., Phase Partitioning Of Ammonia And Ammonium In A Multiphase System Studied Using A New Vertical Wet Denuder Technique, Atmospheric Environment, vol. 25, no. 3, p. 365-371 (1998). Recently, Rosman et al. presented a perfluoralkoxy (PFA) TEFLON, parallel-plate, wet denuder with a polyester lining for wettability. See Kai Rosman et al., Laboratory And Field Investigations Of A New And Simple Design Of The Parallel Plate Denuder, Atmospheric Environment, vol. 35, pp. 5301-5310 (2001). Such known wet denuders work well and are near-quantitative collectors of common soluble trace gases in the atmosphere at least up to sampling rates of 5 L/min.
A major shortcoming of known wetted denuders is that they must be operated strictly in the vertical orientation or the horizontal orientation for which they are designed. Any significant tilting, even for a limited time, can cause collector liquid to get into the air-sample inlet line causing subsequent analyte losses. For example, it would be of advantage to use wetted denuders during airborne sampling. Although an aircraft is usually on level flight during airborne sampling, the use of known wet denuders during aircraft-based airborne sampling is problematic. The aircraft may pitch, roll and/or yaw during a banked turn, while changing altitude, and/or when the aircraft subjected to turbulence, thus causing the denuder to tilt. In the event that one of the above wet denuders is used, analyte loss may occur when the collector liquid spills into and/or enters the air-sample inlet as the denuder is tilted.
In tubular-membrane-based denuders, often referred to as diffusion scrubbers, the sampled gas flows on one side of a membrane and the collector liquid flows on the other side. More commonly, hydrophobic membranes have been utilized in such tubular membrane-based denuders. Even when hydrophilic ion exchange membranes are used, for example, membranes of the type sold under the trademark NAFION sold by E. I. Du Pont De Nemours And Company of Wilmington, Del., the membrane thickness and water permeability through it are generally not high enough to keep the surface of the membrane that is in contact with the gas flow “wet”. Moreover, ionogenic gases cannot be conveniently collected with ion exchange membranes as the analytes of interest are either too tightly bound to the membrane or they are Donnan-excluded. Recently introduced “artificial lung” samplers of the type described by Sakamoto et al. include a large number of hollow fibers, for example, approximately 10,000 polyolefin hollow fibers; these are likely to suffer from extensive particle deposition. See K. Sakamoto et al., Development Of An Automatic Continuous Analyzer For Water-Soluble Gases In Air By Combining An Artificial Lung With An Ion Chromatograph, Atmospheric Environment, vol. 36, pp. 441-448 (2002).
While there are advantages to known membrane based collectors, thus far, the maximum flow rate at which diffusion-scrubber based collectors can continuously remove atmospheric gases of interest has been quite modest and it would have been impractical to use them to remove gases ahead of a particle analysis system. In the recent past, particle analysis systems have typically relied on sampling rates of 4-5 L/min. Under these conditions, the ion chromatographic (IC) instrumentation used at the back end is sufficiently sensitive that the attainable limits of detection (LODs) were not only far below what was required for ambient air measurement, they were blank-variation limited. The present generation of IC instrumentation have become even more sensitive. With the current IC instrumentation, sampling rates of 1 L/min are sufficient to achieve low ng/m3 LODs for most aerosol phase soluble ionic analytes of interest.
What is needed is a denuder for collection and removal of soluble atmospheric gases that overcomes the above and other disadvantages of known wet denuders and diffusion scrubbers.